Determination of water permeability in stress-free lipid membranes through time-resolved small-angle neutron scattering.

Accurate quantification of water permeation across lipid membranes is challenging, with reported permeability values varying widely due to methodological differences and sample heterogeneity. Here, we show that time-resolved small-angle neutron scattering with rapid contrast variation enables direct non-invasive measurement of water permeation kinetics and membrane structure under iso-osmotic conditions. Our results indicate much faster water permeation in gel-phase bilayers than previously reported, likely due to improved structural control and the removal of multilamellar artefacts. Molecular-dynamics simulations agree in activation energies, though absolute permeabilities are higher in silico, highlighting methodological sensitivities. Our integrated approach offers new possibilities for future studies of diffusive solute transport across lipid membranes.